Knightly orders work for the rulers of certain settlements.
The Orders are:
Knights of the Dragon – Daggerfall
Knights of the North – Northpoint
Knights of the Newt – Evermore
Knights of the Wolf – Farrum
Knights of the Flame - Jehanna
Knights of the Owl – Camlorn
Knights of the Rose – Wayrest

Here's my idea. Seeing as the Bretons in High Rock are more interested in becoming legendary heroes through questing, why not use the Knights order factions as the MQ? An series of spectacular quests across High Rock would fit the province's theme much better than some generic war campaign, or assassination plot. Rather than a single story quest we could have multiple stories, 1 or 2 for each city. Like Imperial guilds but on a much larger scale.

The only foreseeable problem would be the fact that lore states a player can only join one order, but I imagine this could be amended: the player can freelance in all of the orders, but can NOT become a "lord" or "duke" through a knightly order unless he leaves all the others. So a player could do all the quests for all the orders, and decided he enjoyed Evermore's order the most, and wants to take over that order. If so, he has to resign from the other orders to recieve his new title, and a keep/fort/manor in the corresponding orders political territory.

Quest ideas:
Dragon: perhaps the most important order, quests could involve putting down a rebellion, hunting down some bay pirates, plundering a barrow due to some ghostly activity.
North: the Eltheric Coast is a harsh, inhuman regions where even the locals dare not venture at night, so this order could include some quests involving necromancers and some sort of army of the dead, if applicable.
Newt: Bog-friggen-monsters, the Eeir Moor could have a real English mysteriousness to it, people could be going missing, human sacrifices are being made, the Newts have to put a stop to it.
Wolf: It'd be too cliche to include werewolves here... But werebears would be cool, and somewhat ironic.
Flame: No idea.
Owl: No idea.
Rose:I'd like to see some skirmishes with the Orcs.

I know it was an example for the ranks but you could use those of Daggerfall. Lord or Duke would make you a noble! (However, a title could be the reward to the main quest.)

One solution for the main quest could be you would have a different point of view of the same events depending of the Order you chose. Each side having its own interests and its own understanding of the situation. It would fit the habit for political intrigues of the Bretons!
As High Rock is divided between Wayrest and Daggerfall, the Knights of the Dragon and the Order of the Rose are okay to join. You could also imagine a third main knightly order for Orsinium (Knights of the Fang?) trying to gain in respectability so their country can be accepted as a true kingdom. For the lesser orders, make them vassals of Wayrest or Daggerfall and depending the Kingdom you choose, you can help/fight them. And instead of knight Orders, Orsinium could have tribes scattered in High Rock you have to help/protect. From a gameplay point of view, they're just quest givers with conditions but you could give them whatever background you want.

You could also use the events of Daggerfall as a starting point for a plot. For example, with the Order of the Raven. After the Warp in the West and the fall of the kingdom of Dwynnen, the Order refuse to submit to Wayrest or Daggerfall and became illegal. Outlawed, they still continued to protect the region for a time but with the years and the new generations, their honor faded and they turned bandits. They're dangerous enemies because they are trained and equipped like knights. Now they're taking refuge in the ruins of their old fortress. The player could fight them (as spawning opponents and finally besieging their hideout) or join them to became a bandit too or restore their ancient glory. He could also repair the fortress through a series of quests. It would make an interesting development for warrior-oriented players. Add a good background and you could say they're no more a knightly order but more a legendary group of bandits/heroes so it shouldn't conflict with the main quest if you join them.